Too Small For Cardinal Tetras?

Leopard Danios and Zebra Danios are the same species.  They are just different colour variations, so you did the right thing in the CC.

Cardinals need to be in groups of at least 6, and that overstocks your tank leaving no room for your danios.

Danios need to be in groups of at least 4 , and that overstocks your tank leaving no room for your cardinals.

Also, both fish need more room than the 40 odd cm in your tank; they are active fish.

There is little that you could actually keep in a tank so small.  The only thing I could come up with through the Community Creator was Endlers.  You could keep a group of Endlers, but make sure they are all male else they will breed.

I know this isn't what you wanted to hear but...  The secret to this, and something to bear in mind for the future, is to research the fish you are going to keep in your tank before you buy the tank.

Hi Jonny,

Welcome to the forum
You have discovered the fun of fishkeeping!  Think about which fish you like, spend ages trying to narrow your favourites down, make your final decision.....then find out you can't have them because...tanks too small, water is too hard/soft, they can't be kept together because they will mate/fight/eat each other! 
Steve raises a very good point, if possible, research the fish you would like first, then get the appropriate tank..too late for you, I know.  I had the same problem.  My tank was a Christmas pressie so I had no input on size etc, (60L) but had an idea of the fish I wanted to keep. Of course, after using the CC and getting advice from the good folks here, it became clear that I wasn't going to be able to have all the ones I wanted.
I think though, that it makes you choose more carefully and have a better understanding of your tank because you have to know your chosen fish's needs and your tank, water supply limitations.  Not much help, I know. Your other option is to do as Sue suggests and treat yourself to one of the beautiful betta fish available, you would also be doing one a huge kindness by rescuing him from the jam jar environment that he undoubtedly has found himself in.
Then, save up.....and get another tank that's a bit/lot bigger depending on your chosen fish's needs!   There does appear to be a well known illness among fishkeepers.....MTS.  It stands for multiple tank syndrome....and be warned, it is extremely infectious

You will get any help you need here, everyone on this site is really helpful, knowledgable and friendly
Hope this hiccup doesn't put you off keeping fish, good luck!

Resa